Known load generators such as nuclear power plants and generators with stochastic, intermittent energy sources such as wind turbines and solar panels, generate excess electrical power during times of low power demand. Large-scale electrical energy storage systems are a means of diverting this excess energy to times of peak demand and balance the overall electricity generation and consumption.
EP1577548 that describes a thermoelectric energy storage (TEES) system that converts excess electricity to heat in a charging cycle, stores the heat, and converts the heat back to electricity in a discharging cycle, when necessary. Such an energy storage system is robust, compact, site independent and is suited to the storage of electrical energy in large amounts. Thermal energy can be stored in the form of sensible heat via a change in temperature or in the form of latent heat via a change of phase or a combination of both. The storage medium for the sensible heat can be a solid, liquid, or a gas. The storage medium for the latent heat occurs via a change of phase and can involve any of these phases or a combination of them in series or in parallel.
The round-trip efficiency of an electrical energy storage system can be defined as a percentage of electrical energy that can be discharged from the storage in comparison to the electrical energy used to charge the storage, provided that the state of the energy storage system after discharging returns to its initial condition before charging of the storage. Thus, in order to achieve a high roundtrip efficiency, the efficiencies of both modes should be maximized inasmuch as their mutual dependence allows.
All electric energy storage technologies inherently have a limited round-trip efficiency. Thus, for every unit of electrical energy used to charge the storage, only a certain percentage is recovered as electrical energy upon discharge. The rest of the electrical energy is lost. If, for example, the heat stored in a TEES system is provided through resistor heaters, it has approximately 40% round-trip efficiency. The roundtrip efficiency of the TEES system is composed of the charging efficiency and the discharging efficiency.
The roundtrip efficiency of the TEES system is limited for various reasons rooted in the second law of thermodynamics. The first reason relates to the coefficient of performance of the system. When the system is in the charging mode, its ideal efficiency is governed by the coefficient of performance (COP). The COP depends on the temperatures of the cold side (Tc) and the hot side (Th) as given by
  COP  =                    T        h                              T          h                -                  T          c                      .  
Thus, it can be seen that the COP of a heat pump declines with increased difference between input and output temperature levels. Secondly, the conversion of heat to mechanical work in a heat engine is limited by the Carnot efficiency. When the system is in the discharging mode, the efficiency (η) is given by
  η  =                              T          h                -                  T          c                            T        h              .  
Thus, it can be seen that efficiency increases when the cold side temperature decreases. Thirdly, any heat flow from a working fluid to a thermal storage and vice versa is dependent on a temperature difference in order to happen. This fact inevitably degrades the temperature level and thus the capability of the heat to do work.
Known industrial processes can involve provision of thermal energy and storage of the thermal energy. Examples are refrigeration devices, heat pumps, air conditioning, and the process industry. In solar thermal power plants, heat is provided, possibly stored, and converted to electrical energy. However, all these applications are distinct from TEES systems because they are not concerned with heat for the exclusive purpose of storing electricity.
The charging cycle of a TEES system can also be referred to as a heat pump cycle and the discharging cycle of a TEES system can be referred to as a heat engine cycle. In the TEES concept, heat should be transferred from a hot working fluid to a thermal storage medium during the charging cycle and back from the thermal storage medium to the working fluid during the discharging cycle. A heat pump should work to move thermal energy from a cold source to a warmer heat sink. Since the amount of energy deposited at the hot side, i.e. the thermal storage medium part of a TEES, is greater than the compression work by an amount equal to the energy taken from the cold side, i.e. the heat absorbed by the working fluid at the low pressure, a heat pump deposits more heat per work input to the hot storage than resistive heating. The ratio of heat output to work input is called coefficient of performance, and it is a value larger than one. In this way, the use of a heat pump can increase the round-trip efficiency of a TEES system.
The charging cycle of a known TEES system includes a work recovering expander, an evaporator, a compressor and a heat exchanger, all connected in series by a working fluid circuit. Further, a cold storage tank and a hot storage tank containing a fluid thermal storage medium are coupled together via the heat exchanger. While the working fluid passes through the evaporator, it absorbs heat from the ambient or from a thermal bath and evaporates. The discharging cycle of a known TEES system includes a pump, a condenser, a turbine and a heat exchanger, all connected in series by a working fluid circuit. Again, a cold storage tank and a hot storage tank containing a fluid thermal storage medium are coupled together via the heat exchanger. While the working fluid passes through the condenser, it exchanges heat energy with the ambient or the thermal bath and condenses. The same thermal bath, such as a river, a lake or a water-ice mixture pool, is used in both the charging and discharging cycles.
There is a need to provide an efficient thermoelectric energy storage having a high round-trip efficiency, while minimising the system costs involved.